Pore-type burner with silicon-carbide porous body

ABSTRACT

A porous burner is configured for burning a fuel-air mixture and to generate a hot flue gas. The burner has a housing and porous material in the housing. The porous material is formed of silicone carbide that is resistant to high temperatures and facilitates combustion. The porous body in the housing is formed of a siliconized carbon fabric with a regular, uniform structure. The shape of the silicon carbide fabric deviates from a planar surface, for example with corrugations, and a plurality of such fabric sections are laid on top of one another.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuing application, under 35 U.S.C. § 120, of copending international application No. PCT/EP 2004/003968, filed Apr. 15, 2004, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German patent application No. 103 17 857.0, filed Apr. 16, 2003 and German patent application No. 10 2004 006 824.0, filed Feb. 11, 2004; the prior applications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a pore-type burner for burning a fuel/air mixture for the purpose of generating a hot flue gas. The burner includes a housing in which a pore material consisting of porous, high-temperature-resistant silicon carbide (SiC) is provided for combustion.

Such a pore-type burner is employed, for example, in order to apply a hot stream of flue gas to a steam superheater. The steam arising in the steam superheater has high temperatures and is under intense pressure. The energy stored in the steam can then be made usable in the form of mechanical or electrical energy, for example as a result of decompression in an expansion engine for the purpose of driving a generator. The hotter the steam and the higher the pressure, the better the efficiency of such machines. Correspondingly, it is necessary for the stream of flue gas to exhibit temperatures that are as high as possible. Typical temperatures lie within the range between 850° C. and 1400° C.

The pore-type burners for generating a hot stream of flue gas differ, in particular, from a pure radiation burner in which only the radiant heat of the burner is utilized and the flue gas arising is drawn off as a secondary product via a chimney or an exhaust-air pipe. Such radiation burners are, for example, artificial open fires or radiation burners for the purpose of drying lacquer coatings. Although the radiant heat of a pore-type burner can also be utilized, the significant portion of the energy transferred to the steam-generator comes from the flue gas.

A pore-type burner for the combustion of a fuel/oxidizing-agent mixture is known from German patent DE 199 39 951 C2. The pore-type burner is filled with globular filling materials. The size of the pores that arise is determined by the size of the filling materials. The prior art pore-type burner is designed in such a way that an excessive temperature in the reaction chamber is avoided by means of an additional cooling gas.

German patent DE 195 27 583 C2 describes a pore-type burner, which contains porous material that exhibits spatially contiguous cavities which are formed from a packing consisting of heat-proof wire material, foil material or sheet-metal material. A defined flame zone forms in these cavities. The material is not suitable for high temperatures.

Moreover, pore-type burners are known—for example, from U.S. Pat. No. 5,890,886—which are filled with a ceramic that is formed with a plurality of cavities. Other foam ceramics, metallic foams or metallic sponges are also known, from German published patent application DE 196 21 638 A1 for example. These foams or sponges have the disadvantage that they are expensive to manufacture. In addition, they are very sensitive to mechanical and thermal loads. They tear or crack in the event of excessive loading, resulting in diminished performance and increased emission of noxious substances.

German published patent application DE 198 47 042 A1 describes a highly porous burner mat, which consists of metallic or ceramic fibers that are welded to one another in irregular structures. The mat is provided with holes, through which the gas flows. Regions of varying flow velocities arise, by virtue of which an irregular carpet of flame arises which lifts off from the surface of the mat.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a porous burner with silicon-carbide porous body which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides a pore-type burner that exhibits uniform combustion and that has a pore structure which is capable of being directly influenced in the production process.

With the foregoing and other objects in view there is provided, in accordance with the invention, a pore-type burner for burning a fuel/air mixture for generating a hot flue gas, comprising:

a housing;

a porous body with pore material formed of porous, high-temperature-resistant silicon carbide disposed in said housing, said pore material being formed of siliconized woven carbon fabric disposed in an ordered, regular structure.

In other words, the objects of the invention are achieved by the porous body comprising siliconized woven carbon fabric that is/are disposed in an ordered, regular structure. The invention is based on the perception that the properties of a pore-type burner can be influenced if the pore structure is capable of being produced for a specific purpose. An interweaving of the hard and brittle material silicon carbide is not possible. However, by siliconizing a suitably shaped woven carbon fabric it is possible to create an appropriately configured woven-fabric structure consisting of SiC. The siliconized woven fabric is capable of being produced inexpensively. It withstands mechanical and thermal loads very well. The mesh width and planar shape of the woven fabric are just as individually adaptable as are its size and contours, so an optimization of the properties of the burners is possible when use is made of materials of such a type by way of a porous body for pore-type burners.

In one configuration of the invention the woven fabric consisting of silicon carbide has a shape diverging from a plane surface. A plurality of pieces of woven fabric can then be arranged in layers on top of one another. In this way, a three-dimensional configuration is created with which the pore-type burner is capable of being filled, without additional spacers or the like.

The woven fabric may be shaped in an undulating manner, i.e., with a rounded corrugation. But other shapes are also possible, such as a profile that is sawtooth-shaped or box-shaped in cross-section. Then, in order to obtain a small pore size, on the one hand the parameters of the woven fabric may be kept small, and on the other hand the wavy shape may then be composed of a plurality of small undulations.

The woven fabric may consist of completely siliconized fibers. But for some applications it may also be sensible for the woven fabric to be partially siliconized and to contain a core consisting of pure carbon.

In a particularly preferred configuration of the invention, the ordered structures are designed in such a way that zones of varying porosity are formed. In this case the porous body of the burner may be designed in two or more zones of varying pore size. The inlet-side part of the porous body then exhibits a smaller pore size than the outlet-side porous body. With this configuration the flame forms in the coarse-pored zone, whereas a mixing and preheating of the fuel/air mixture take place in the fine-pored zone. This results in a particularly low noxious-substance content of the flue gas in the course of combustion of the customary fuels such as natural gas, gasoline or such like. The pore size can be realized particularly well by virtue of the selected woven fabric and its arrangement—such as, for example, its stacking.

In an alternative configuration of the invention the fine-pored part is produced from materials forming conventional pores, whereas the coarse-pored part consists of siliconized woven carbon fabric. The material of the fine-pored part is preferably poorly conducting, so that a transfer of heat out of the combustion zone into the premixing zone is avoided. In this way, a backfiring of the flames is prevented.

The axes of curvature of the undulations of a piece of woven fabric may lie in a plane, and the pieces of woven fabric may be arranged above one another in such a manner that the projections of the wave normals onto such a plane defined by the axes of curvature extend perpendicular to one another. The wave normals then preferably each form an angle of approximately 45° relative to the direction of flow of the flue gas. A wave normal here is the perpendicular to a wavefront; it lies in the plane defined by the axes of curvature. With this configuration of the invention the pore structure is formed from stacked undulating SiC mats. In this case the individual planes are arranged rotated by an angle of approximately 90° in relation to one another. This arrangement is particularly favorable for the combustion behavior of the burner. The structure that is flowed through in such a way is designated as a static mixer. In the process, the fuel and the combustion air are mixed with one another in such a way that the fuel is burnt in a particularly low-emission manner and completely.

The housing of the burner is preferably provided with an insulating layer. In this way, an undesirable convective transfer of heat through the housing into the periphery of the burner is avoided.

Alternatively, the wall of the housing may be flowed through by a cooling medium which is either conducted away separately into the environment or mixed with the hot flue gas in the outlet region of the burner.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in a pore-type burner with silicon-carbide porous body, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective representation of a pore-type burner;

FIG. 2 shows a detail from a piece of woven fabric shaped in a corrugation and consisting of silicon carbide;

FIG. 3 is a longitudinal section through a schematically represented pore-type burner; and

FIG. 4 is a section along the line IV-IV in FIG. 3 and shows the outlet of a pore-type burner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a schematic representation of a pore-type burner 10. The pore-type burner consists of a housing 12, into which a fuel-gas/air mixture is introduced. The direction of flow of the inflowing gas is represented by the arrows 14. In the housing 12 a plurality of pieces of woven fabric 16 are arranged in layers on top of one another. In a first zone 18 the pores are smaller, and in a second zone 20 the pores are larger. The porous material of the first zone 18 is not represented. In the second zone an oxidation takes place in the pores without genuine formation of flame. In the process, hot flue gas arises which is represented in FIG. 1 by arrows 22. The flue gas is utilized in order to heat a steam-generator. In this case there is the possibility of arranging the steam-generator within the radiation field of the pore-type burner 10, so that not only the heat transferred by the flue gas but, in addition, the radiant heat is also utilized.

The pieces of woven fabric 16 are represented again in FIG. 2 in detail. They consist of a substantially rectangular, net-like woven fabric. A plurality of these pieces of woven fabric 16 are arranged in layers one on top of the other. Each piece of woven fabric 16 is curved in an undulating manner, or corrugated, about an axis of curvature 37. The pieces of woven fabric are arranged in layers on top of one another in such a way that the crests 24 and troughs 26 of the curvatures are always situated on top of one another, offset alternately by substantially 90 degrees. This is evident in FIG. 3. For instance, the piece of woven fabric 30 rests on the piece of woven fabric 28, offset by 90 degrees. The pore-type burner is filled up completely with the pieces of woven fabric 16. As a result, a pore structure forms that allows a particularly good, uniform evolution of flame. The porous body is flowed through by the fuel/air mixture parallel to the planes of the individual layers of woven fabric and in the direction of the bisectors 34 of the angle of rotation between the wave normals 35 and the wave normals 39 of the layers.

In the present case the pore-type burner 10 has a rectangular cross-section and is therefore also filled with rectangular pieces of woven fabric 16. Of course, if the pore-type burner 10 has a differently shaped cross-section, the shape of the pieces of woven fabric is also adapted correspondingly.

Moreover, a coolant flows through the housing 12 of the pore-type burner. The cooling air in this case is fed separately into a cooling duct 38 (FIG. 4) of the housing 12 and is mixed with the flue gas at the outlet 40.

The pore size can be influenced through the size of the woven-fabric meshes 32, the radii of curvature of the wave troughs and wave peaks, and the number of curvatures per piece of woven fabric. In the present exemplary embodiment the pore size is smaller in zone 18 (FIG. 1) and larger in zone 20.

The pieces of woven fabric consist of silicon carbide. Silicon carbide is a carbidic ceramic material and as such is not weavable. For the purpose of producing woven fabric of such a type, use is therefore made of a woven carbon fabric which is brought into the appropriate shape and then siliconized. Various processes are suitable for the purpose of siliconizing. In the case of the liquid siliconizing process, molten silicon is infiltrated into a porous substrate consisting of carbon-fiber-reinforced carbon (C/C) and is caused to react directly with the carbon of the matrix so as to form SiC. The process is known and, for example, described on the Internet under http://www.fz-juelich.de/iwv/iwv1/index.php?index=8 and therefore does not need to be elucidated in any detail.

After this process the siliconized pieces of woven fabric 16 are stiff and can be inserted into the burner without further change of shape. The material is resistant to high temperature. The production process for planar SiC structures is inexpensive, compared with sponge-like ceramic bodies, and the mechanical and thermal load-bearing capacity is significantly higher in comparison with ceramic sponges. 

1. A pore-type burner for burning a fuel/air mixture for generating a hot flue gas, comprising: a housing; a porous body with pore material formed of porous, high-temperature-resistant silicon carbide disposed in said housing, said pore material being formed of siliconized woven carbon fabric disposed in an ordered, regular structure.
 2. The pore-type burner according to claim 1, wherein said woven fabric consists of silicon carbide having a shape diverging from a plane surface, and a plurality of pieces of woven fabric are arranged in layers on top of one another.
 3. The pore-type burner according to claim 2, wherein the woven fabric has an undulating shape.
 4. The pore-type burner according to claim 1, wherein said woven fabric consists of completely siliconized fibers.
 5. The pore-type burner according to claim 1, wherein said woven fabric is partially siliconized and contains a core consisting of pure carbon.
 6. The pore-type burner according to claim 1, wherein said ordered structures are configured to form zones of varying porosity.
 7. The pore-type burner according to claim 3, wherein said undulating shape is defined with axes of curvature and said axes of curvature of the undulations of a piece of woven fabric lie in a plane, and the pieces of woven fabric are arranged on top of one another with respective projections of the wave normals onto such a plane defined by the axes of curvature extend substantially perpendicular to one another.
 8. The pore-type burner according to claim 7, wherein the wave normals are disposed to form an angle of approximately 45° relative to a direction of a flow of the flue gas. 